The present invention relates to automated or semi-automated transmission systems with a multi-ratio transmission and a gear-engaging mechanism for engaging the ratio at which the transmission is to operate.
In semi-automated and fully automated transmission systems, the gear ratio for setting the vehicle in motion from stand-still is in many operating situations selected automatically after the drive-mode lever has been moved. In fully automated transmissions, the selection of the gear ratio may be performed automatically by a control unit, so that no intervention by the driver is required at least in one operating mode of the transmission. The transmission can also have another operating mode in which the gear-ratio selection can be set by the driver. In transmission systems with an automated clutch, however, the take-off gear ratio is selected manually by moving the drive-mode lever.
The gear-engaging mechanism may be based on a conventional manual selector mechanism which, as a rule, has a selector member that is engageable with three shift rails. The selector member is moved in a first direction to engage one of the shift rails and in a second direction to move the engaged shift rail to actuate a gear. The shift rails normally control two gear ratios, movement of the shift rail in one direction engaging one gear ratio and movement of the shift rail in the opposite direction engaging the other gear ratio.
The automatic gear-engaging mechanism of the present invention has a pair of actuators. One actuator serves to move the selector member in the first direction, and the other actuator serves to move the selector member in the second direction. The actuators may be, for example, pneumatic or hydraulic cylinders or electric motors.
In accordance with International Patent Specification WO97/05410, the contents of which are hereby expressly incorporated by reference in the present disclosure, a pair of double-acting hydraulic cylinders are used to move the selector member; in an X-direction to engage an appropriate shift rail; and in a Y-direction to move the shift rail to engage gear. A pair of potentiometers are associated, respectively, with each of the actuators to sense a position of the actuator. The actuators are controlled by a closed-loop control circuit by means of feedback from the potentiometers to move the actuators to predetermined positions for each gear ratio of the transmission.
With systems of this type, a malfunction of one or both potentiometers may prevent the engagement of an appropriate gear ratio. Failure of the position-measuring potentiometer or the associated wiring can be detected if the measurement falls outside the normal range of operation. Normally, the interfacing circuit between the potentiometers and the electronic control unit will generate a voltage outside the normal potentiometer range in the event of an open-circuit failure of any one of the connections between the electronic control unit and potentiometer. The ranges outside the normal range of operation are referred to as guard bands.
It is, however, more difficult to detect a potentiometer failure in a case where the interface circuit generates a voltage within the normal operating range. This failure mode may result from increased resistance of the potentiometer track and the wiper. It is possible to detect failures of this type while the vehicle is in motion if it is possible to determine which gear is engaged based on a comparison of the vehicle speed and engine speed, if there is no clutch slippage. The actual position indicated by the potentiometers may thus be checked against the predetermined position of the potentiometers for the gear engaged.
However, when taking off from stand-still with the transmission system automatically selecting a start-up gear, typically first, second or reverse, it is not possible to check the potentiometers in this manner. Thus, if a potentiometer fails at start-up, it is possible in extreme circumstances that reverse gear (xe2x80x9cRxe2x80x9d) may be selected instead of first gear or vice versa. This may result in a potentially hazardous situation if the vehicle starts off in the opposite direction to that expected.
It is therefore the object of the present invention to solve the aforementioned problem inherent in state-of-the-art transmission systems of the kind described above.
According to one aspect of the present invention, the foregoing problem is solved for the case of an automated transmission system with a multi-ratio transmission and a gear-engaging mechanism. The latter has a selector member and a pair of actuators, subsequently referred to as first and second actuator. The first actuator serves to move the selector member in a first direction to select a gear ratio, and the second actuator serves to move the selector member in a second direction to bring the selected gear ratio into engagement. Each actuator is equipped with a position sensing means to provide a position signal corresponding to the gear ratio engaged. The transmission has forward and reverse start-up gears in which the vehicle may be started from stand-still. The position of the selector member at which the start-up gears are selected is located at the end positions of the range of movement of the selector member. The transmission system also includes a control means to automatically select a start-up gear to start the vehicle from stand-still. In the start-up phase of the vehicle, the control means drives the first actuator to the end position of the range of selector movement that corresponds to the location of the selected start-up gear and then drives the second actuator within its range of movement in the direction corresponding to the engagement of the selected start-up gear.
In accordance with the present invention, the start-up gear can be engaged without being referenced to the predetermined start-up gear position. The actual position indicated by the position-sensing means when the start-up gear is engaged in the manner described above may then be compared to the predetermined position. If a discrepancy is found in this comparison, this would indicate a failure condition in one or both position-sensing means. Start-up from stand-still may then be inhibited if a malfunction is detected in this manner.
In a practical embodiment of the invention, the control means is configured such that, at start-up from stand-still, the control means will drive the first and second actuators for a predetermined time period towards end positions of their respective movement ranges in a direction corresponding to the selected start-up gear.
It is preferred if one end position of the range of movement of the selector member in the direction controlled by the first actuator corresponds to the first and second gear ratios and the opposite end position of the range of movement corresponds to reverse gear.
In a preferred configuration of the control means, when a start-up gear has been selected, the control means will compare actual position signals delivered by the position-sensing means to predetermined position data and, if a discrepancy is found between the actual position signals and the predetermined position data, the control means will inhibit start-up from stand-still.
Further in the aforementioned preferred configuration of the control means, if a discrepancy is found between the actual position signals and the predetermined position data, the control means will register a failure condition.
In another preferred embodiment, the control means is configured such that, if a discrepancy is found between the actual position signals and the predetermined position data in the direction controlled by the second actuator, the control means will make further attempts to bring the selected start-up gear into engagement, before the control means will proceed to register a failure condition and/or inhibit start-up from stand-still.
It is of practical benefit if the position-sensing means includes a potentiometer.
In a preferred embodiment of the invention, once the vehicle is in motion, the control means will control engagement and disengagement of the gear ratios by using feedback coming from the position-sensing means through a closed-loop control system.
In a further development of the embodiment just mentioned, once the vehicle is in motion, the control means will perform a comparison between engine speed and vehicle speed and thereby determine which of the gear ratios is engaged. If a discrepancy is found between the actual position signals and the predetermined position data for the gear ratio engaged, the control means will register a failure condition.
In another preferred embodiment, if a position sensor generates a position signal which is out of the normal range of movement of the selector mechanism, the control means will register a failure condition.
Under an advantageous concept of the invention, the control means will inhibit the vehicle from starting up from stand-still if the control means has registered a failure condition.
With preference, the actuators are pneumatic or hydraulic actuators.
It is advantageous if during start-up from stand-still, the control means will reduce the pressure level supplied to the actuators in order to avoid damage to the selector member.
In a further advantageous embodiment of the invention, the actuators are electric motors.
In the embodiment just mentioned, it is advantageous if, during start-up from stand-still, the control means reduce the energizing current to the electric motors in order to avoid damage to the selector member.
The novel features that are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiments with reference to the accompanying drawing.